The present invention relates to electromagnetic flow meters. However, aspects of signal processing techniques disclosed herein may be more broadly applied. The operating principles of Electromagnetic Flow Meters are well known, discussed for example in GB-A-2,380,798.
Where the sensing electrodes are in contact with the fluid, due to electrochemical or other effects, a DC potential is usually present across the electrodes even when there is no coil excitation, i.e. no field. That component of the signal is independent of the flow. This inhibits the ability to determine the flow in a static fashion. To overcome this some form of dynamic excitation to the coils is typically provided in order to generate a dynamic component at the electrodes that can be differentiated from the background DC (or slowly varying) bias signal. This dynamic signal is normally pulsed DC or an AC signal.
The bias signal will not generally be static. It may drift randomly with time, flow and temperature. In some applications, particularly for non-homogeneous fluids with inclusions, the signal may contain large amplitude decaying exponential components, for example in paper pulp and slurry applications, as discrete charged particles occasionally touch the electrodes, changing the voltage instantaneously and this voltage then discharges exponentially to the (drifting) baseline.
The energy can be spread across a wide frequency range but most applications have significant low frequency noise and this is often more problematic due to the nature of the signal processing.
Therefore, one way to obtain flow measurements which are less susceptible to such effects might be to use a high frequency excitation, above most of the noise effects. It is found that a frequency above about 1 kHz would for most practical purposes exclude most bias effects. However, using such a frequency would introduce its own problems. Most notably, the magnetic circuit of the flow meter is less stable at the higher frequencies. One factor that contributes to this is that the losses in the magnetic circuit, which become more significant at higher frequencies, are quite temperature dependent. Thus, for a given excitation, the field strength generated may vary unpredictably. This can be exacerbated because the excitation coils are usually positioned outside a steel shell of the flow meter. A search coil can be used to measure the actual field generated but this adds significant cost and manufacturing difficulties.
The ‘roll off’ of the sensor head at these higher frequencies leads to an uncertainty in the sensitivity of the sensor and to variations in the phase between the field (and hence the electrode signal) and the drive current. It is known to use a moderate frequency (e.g. 70 Hz) sinusoidal excitation and to demodulate the electrode signal synchronously with the excitation signal. Quite apart from other considerations, the phase angle at which the demodulation is (should be) carried out is not constant and requires either manual or automatic adjustment.
To summarise the problems a lower drive frequency can give much better sensor stability but measurements are more easily corrupted by the bias drift and the effect of inclusions in the fluid. A further important problem is that a low excitation frequency limits the rate at which new measurements can be updated—it gives a low flow measurement bandwidth. A higher frequency assists in distinguishing wanted signals from unwanted and also allows a more acceptable higher flow measurement bandwidth but the sensor characteristics will not be so precisely defined and are less stable. The frequency chosen is therefore normally a compromise for a particular set of circumstances.
These problems have been previously addressed and it has been proposed in the assignee's earlier filed UK patent application no. 0116168.6 to excite a meter with two frequencies simultaneously or quasi-simultaneously. Each frequency component is separately extracted and a combined measurement obtained in such a way as to enhance the better properties of each measurement. A drawback is that measurement rate is still limited by the lowest frequency, as explained in more detail below.